This invention relates to a switched-mode power supply, comprising: a series arrangement of a controllable switching element and an inductive element, connected to receive an input voltage, means for rendering the controllable switching element conductive during an on period, so as to produce a current through the series arrangement, which current increases from a minimum value at the beginning of the on period to a maximum value at the end of the on period, and for rendering the controllable switching element non-conductive during an off period.
Such a switched-mode power supply is known from the International Application WO 97/30574. In said known power supply the inductive element is the primary winding of a transformer and the controllable switching element is a switching transistor. The transformer further has a secondary winding which supplies an output current to a battery to be charged and/or another load via a rectifier diode. A problem with this known load is the output current limitation in the event of a short-circuit of the load to be powered. A short-circuit can occur in the case of a defect in the load or in the battery to be charged. In rechargeable electrical apparatuses, such as shavers, tooth-brushes, vacuum-cleaners, telephone sets and the like, which are placed in a stand or holder in order to charge the battery, the risk of a short-circuit is also substantial because the contact pins of the power supply can inadvertently come into contact with a conductive object. The same can happen if the power supply is incorporated in a mains voltage adapter or power plug which supplies charging current to the rechargeable apparatus. The outlet of the adapter can be short-circuited inadvertently.
A customary method of current-limiting, which is also used in the known power supply, is to limit the maximum value Ipmax of the primary current. For this purpose, the primary current is measured and the switching transistor is turned off as soon as the primary current exceeds a certain value. However, in this case the average output current can still run high when the switched-mode power supply starts to operate in the continuous conduction mode, which means that the switching transistor is turned on again before the output current has decreased to zero. The primary current then increases each time from a minimum value Ipmin at the beginning of the on period to the maximum value Ipmax at the end of the off period. When the load to be powered is short-circuited the average output current is approximately equal to n*(Ipmax+Ipmin)/2, where n is the transformation ratio. Since the output is short-circuited the output current can hardly decrease in value during the off period. In that case the maximum value Ipmax is reached almost immediately during the on period, as a result of which the output current rises to approximately n*Ipmax.